The invention relates to a switched-mode power supply as defined in the preamble of claim 1. The invention further relates to a display system using such a power supply. The power supply may be used in small personal communication or information equipment, for example, portable telephones or pagers.
A switched-mode power supply is known from U.S. Pat. No. 5,587,683. The power supply described in this document is used in a liquid crystal display device. The switched-mode power supply comprises converting means for converting an input voltage into an output voltage Vlcd. The output voltage Vlcd is the highest voltage needed for the multiplexed driving of the liquid crystal display device. Furthermore, the switched-mode power supply comprises control means for generating a control signal for the converter in dependence on a feedback signal and a feed-forward signal. The feedback signal is dependent on the output voltage Vlcd and a reference voltage. Furthermore, a bias level generator and a multiplexing circuit are present for generating a row and a column signal from the output voltage Vlcd and the information to be displayed. The row and column signals are used for driving the liquid crystal display. When a new row of the liquid crystal display is selected, a peak current is drawn from the bias level generator and a voltage dip occurs for a short time in the output voltage Vlcd. In order to adapt quickly to changes in the load of the liquid crystal device caused by different states of the row and column signal, the feed-forward signal is derived from a change in the row signal or a change in the column signal. This feed-forward signal is used for improving the feedback regulation of the liquid crystal drive voltage. However, in liquid crystal display devices, chip-on-glass technology may be used for the power supply, control circuits, the bias level generator and the multiplexing circuit. In that case, parasitic track resistance of indium tin oxide (ITO) conductors between these circuits may add an additional time constant to the feedback signal in the Vlcd feedback loop. Due to the transfer characteristic of the ITO tracks on the liquid crystal device, it may take some time before the output voltage is back at the required level. In this situation, the regulation may introduce some lag when the load varies during consecutive rows, which may affect the grey value and color of the liquid crystal display.
It is an object of the invention to provide a switched-mode power supply in which the lag of the output voltage regulation is reduced. This object is achieved by the switched-mode power supply according to the invention as defined in claim 1. When the control means has a priori information when and how the load changes at the beginning of each consecutive cycle, for example, the row periods of a multiplexed liquid crystal display, the feed-forward signal can be retrieved, corresponding to an expected load level in accordance with a suitable switching pattern of the converter. The invention is based on the recognition that the variation of the load during successive cycles exhibits the same pattern during a period which is long compared to the operation period. For example, in certain applications of liquid crystal displays, the information content of the display is constant during a period which is generally much longer than a frame period with which the liquid crystal display is refreshed. The varying load level may be a priori known by, for example, a display sequencer means. Information of the display sequencer means can thus be used for determining the feed-forward signal.
Further advantageous embodiments of the invention are defined in the dependent claims.
A particular embodiment of the invention is defined in claim 2. By using the combination of the feed-forward signal and a feedback signal, an efficient regulation is obtained.
A further embodiment of the power supply according to the invention is defined in claim 3. Generating a feed-forward signal, which is smaller than the predetermined running period of the control signal, ensures a stable and converging regulation of the power supply.
A further embodiment of the power supply according to the invention is defined in claim 4. The switching information of, for example, the subsequent line periods during the frame period of the display can be stored in the circular memory and retrieved by incrementing the read address each cycle.
A further embodiment of the power supply according to the invention is defined in claim 5. By application of the acquisition means, the status information of the varying load level can be acquired, so that an optimal switching pattern of the switched-mode power supply can be obtained for each subsequent cycle.
A further embodiment of the power supply according to the invention is defined in claim 6. By generating the feed-forward signal in dependence on the frame signal, a higher peak current during, for example, a frame inversion of the liquid crystal display can be compensated for.
The invention further relates to a display system.
It is an object of the invention to provide a display system for displaying grey scale and colors with a high multiplex ratio. This object is achieved by a display device according to the invention as defined in claim 8.